For many applications, the capacities of current optical storage devices (CDs, DVDs, or even Blu-ray Discs) are very insufficient. Research is thus being conducted with the aim of achieving storage capacities on the order of one terabyte, whereas current discs offer 50 gigabytes at best.
To this end, in the prior art, optical mass storage devices are already known that use the principle of holography. This principle offers the great advantage of making 3D or volumetric data storage possible, instead of the mere 2D or surface data storage (or possibly multi-surface data storage) afforded by conventional discs of the DVD type. In particular, EP 0 827 621 describes an optical mass storage device having a light-sensitive layer with elementary cells, each of which is constituted of a light guide segment of the optical fiber type. The fibers then store a Lippmann structure constituting an interferogram of data encoded in the wavelengths of a light beam.
FIG. 1 of EP '621 illustrates, during writing of data, that light penetrates into the fiber, passes through a transparent substrate, and reaches a mirror which reflects the light back into the fiber. The light-sensitive material of the fiber is then subjected to the action of two beams that propagate in opposite directions. The interference patterns resulting from interference between the two beams then generate standing waves which inscribe in the light-sensitive material of the fiber a superposition of refractive index stratifications pursuant to the Lippmann effect.
During data reading, the mirror is masked, and the laser emits light having a continuous spectrum. Certain wavelengths are selectively reflected towards the laser and the detector by the Lippmann structure. Detection of that structure thus indicates presence of an information bit. By multiplexing various wavelengths in the fibers, it is then possible to obtain volumetric storage of a plurality of information bits.
It is easy to understand that the drawback lies in the fact that it is necessary to remove or to mask the mirror during reading to read the interference patterns. The technical problem presented is thus the problem of reading or of detecting interference patterns in a holographic mass storage device.
US 2002/150022 discloses using ultra-short pulses for recording information at a determined depth in the thickness of the material. The delay between two counter-propagating pulses defines the depth of recording. Interference patterns are used therein during the method for recording the information, but never during the method for reading the information. In that system, a mirror is present under a light-sensitive medium. That mirror (with the quarter-wave plate) has polarization properties so as to avoid mixing the beam reflected by the mirror with the same beams that encode the data that is read.
US 2003/165746 discloses a structure of a recording medium and a method for writing data on that medium.
EP 1 324 322 discloses the fact that, during writing, a reference beam reflected by a mirror is sent in a direction different from the direction of the reconstructed signal.
None of those publications discloses a step of reading data by homodyne detection.